2-Minute Video: Mangroves don’t love the cold, but relatively mild winters have seen them multiply north of their range. Randall takes a closer look at black mangroves in the salt marshes of Saint Joseph Bay.

Dr. Randall Hughes FSU Coastal & Marine Lab/ Northeastern University

A few years ago, I took my colleague Dr. Ed Proffitt to check out the marshes in St. Joseph Bay. He asked to see mangroves, and I thought he was crazy. Mangroves up here? No way! But we had only been in one Buffer Preserve salt marsh together for a few minutes before I realized that the small “shrubs” that I had previously ignored were actually small black mangroves! And the more we looked, the more we found. They aren’t everywhere, but they can be quite abundant in some places.

Shrubby black mangroves (Avicennia germinans) appear to be an increasingly common site in the marshes of the Saint Joseph Bay State Buffer Preserve.

Mangroves typically occur below the “frost line”, or in areas that don’t experience hard freezes. Lore has it that mangroves have become more common in the northern Gulf of Mexico in recent years due to a series of mild winters. I haven’t been monitoring them long enough to say whether or not there are more now than there were, say, 10 or even 20 years ago, but it’s not hard to see that the ones that are here are successfully reproducing, with small seedlings surrounding the adult trees.

There are even red mangroves lingering around – they are less cold-tolerant than the black mangroves and a surprise to find in our marshes!

I definitely have not seen any significant dieback in the last 5 winters, even when we have had hard freezes. And I would not be surprised if they become more common and abundant as the climate continues to change.

Mangroves in the marsh raise a number of interesting questions. Will they take over? What will that mean for the services these areas provide to people? Will the fishes and crabs that we like to eat become more or less abundant if mangroves dominate over marsh grasses?

Unfortunately, I don’t have the answer to these questions. But I can say that the mangroves that occur in St. Joseph Bay aren’t necessarily “better” at surviving in the northern Gulf than mangroves from farther down south. And why should they be? Well, if a group of mangrove propagules arrived in St. Joe Bay, we may expect that only a subset of them would be able to survive the colder temperatures, and when these propagules grew into adult trees and produced propagules of their own, they should pass that “benefit” to their offspring (the process known as natural selection).

Black mangrove propagules.

How do we we test whether St. Joe Bay mangroves are better equipped to live here than mangroves from down south? We have 2 ongoing experiments where we’ve planted “propagules” (young mangroves that look a lot like seeds) from different locations throughout FL in St. Joe Bay and followed them through time to see which ones survive and grow the best. There’s a lot of variation, but the St. Joe Bay propagules (which were largely the “runts” of the bunch to begin with) did not do as well as propagules from some of the areas down south such as Cedar Key and Cape Canaveral. These results suggest that it doesn’t take a particularly special propagule to survive in the northern Gulf; instead, there probably aren’t just many propagules that make it up here to begin with.

Of course, we’ve only been monitoring these propagules for 1-2 years; maybe the St. Joe propagules have an advantage when they get old / big enough to reproduce. We don’t want to speed up the mangrove take-over, so we’ll remove the seedlings in our experiment before that happens. But we’ll definitely continue to monitor the ones that already made it here on their own accord to see what they do next!

The Guana Tolomato Matanzas National Estuarine Research Reserve (NERR) south of Saint Augustine is where Randall and David have done a lot of their oyster research. There, mangroves mingle with marsh cordgrass. Could salt marshes in St. Joseph Bay (or north Florida in general) one day look like something approximating this?

Music in the video by pitx.

This material is based upon work supported by the National Science Foundation under Grant Number 1161194. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

Dr. Randall Hughes FSU Coastal & Marine Lab

Ed Proffitt with Randall Hughes. If global climate trends continue, mangroves may start to overtake the salt marsh ecosystem along the Gulf coast. What will these new habitats look like?

A few weeks ago, Dr. Ed Proffitt from Florida Atlantic University visited FSUCML to give a seminar here and on campus. Ed and I have collaborated on several proposals, so we used the visit as an opportunity to get out in the field and toss around some new ideas.

Ed has done some really interesting work on the interactions between mangroves and salt marsh plants in Tampa Bay and the Indian River Lagoon, and he wanted to see some mangroves in this area. I recalled having seen a few young red mangroves last year at some of our sites, but none of them survived this past winter (which is why we generally don’t find them around here – they can’t withstand the cold temperatures that we get every few years). However, black mangroves do extend into this portion of the Gulf, and I knew of a place where we may find one or two small ones to look at.

Black mangrove (Avicennia) growing in St. Joe Bay

To my surprise, we found a lot more than one or two! And although they are small (think small shrub, rather than tree), some of them, such as the one shown here, had aerial roots extending out 14-15m, suggesting that they have been around at least 5-10 years (by our best guess).

Avicennia flower. These mangroves are insect-pollinated, and we saw lots of bees buzzing around.

Avicennia propagules growing on the maternal tree.

Also, most of the larger ones had both flowers and propagules (seedlings that are retained on the tree) on them.

Avicennia propagule that has dropped to the ground and started to take root.

As we looked around, we noticed more and more small mangroves in the marsh – probably the seedlings from some of the nearby larger trees – and we even found some of this year’s propagules that were starting to root in the sediment.

As I mentioned, black mangroves are known to grow in the Panhandle and west into Louisiana and Texas, so it really isn’t that surprising that we found them in St. Joe Bay. What is surprising, at least to me, is that they are as abundant as they are in a site where I previously thought there were only a few. Where else may they be in the bay? And are they increasing in abundance each year? What impact do they have on the marsh plants and animals? The questions abound. With our curiosity and Ed’s insight and experience, we are now starting to pursue the answers.